This invention concerns thermoforming apparatus, and more particularly presses used in thermoforming parts from preheated plastic sheet material. Thermoforming is in wide spread use as a manufacturing method, and involves preheating of thin plastic sheet material in an oven and then advancing the same into a thermoforming press having a mold located between the press platens.
The plastic sheet is drawn against the mold surfaces during the forming operation carried out by the application of a vacuum or air pressure causing the sheet material to be drawn against the mold surfaces. Relatively large articles are often manufactured this way and considerable press loads are generated by the fluid pressure forces acting over large areas, which must be resisted by the press platens and their supporting structure.
The forces required also may be increased by the use of plug assist tools in which complementary tooling is carried by one of the platens and driven into the forming surfaces on the other tooling piece. The weight of the tooling supported by the platens thus can become considerable. A large servo motor has sometimes been used, the size of the motor in turn resulting in a relatively slow response time of the platen drive mechanism. The relatively thin wall thickness of the part makes accuracy in the forming process more critical.
Presses used in thermoforming typically involve a pair of columns supporting the platens in their up and down motion and plain bearings are used in the linkages used to drive the platens up and down and to guide the platens on the machine frame. This construction has limitations in that the deflections of the platens under heavy loading is not sufficient to prevent inaccuracies in the thin walled parts formed due to slight tilting and bending of the platens and the structure supporting the platens under the heavy press loads described. In addition, a single large servo motor has considerable inertia increasing the cycle time required. This construction also absorbs a significant portion of the motor power to overcome the frictional losses. Where the apparatus is taxed to its utmost capability, these losses become critical, and the press may not be able to execute the operation required.
Accordingly, it is an object of the present invention to provide a thermoforming apparatus having an improved thermoforming press in which the stability and precision movement of the platens is substantially improved over the prior art thermoforming presses.
It is a further object of the present invention to provide a press for use in thermoforming operations in which the frictional losses are significantly reduced.
It is yet another object of the present invention to provide a servo motor operated platen mechanism in which the response capability of the servo motor drive is improved over the prior art designs for heavier duty presses.